Photocatalytic water splitting has been studied extensively for converting solar energy into clean hydrogen fuel (H2). The development of photocatalysts that can be excited by photons having sufficiently long wavelengths (e.g., greater than 400 nm) is a prerequisite to efficient solar energy conversion and storage. The solar energy conversion efficiencies of the photocatalysts developed to date for direct water splitting has been low, typically, less than 3% under visible light irradiation. However, indirect photocatalytic splitting of water is also possible using sacrificial electron donors. This method of water splitting is viable only if a closed-loop scheme (i.e. a chemical cycle) can be envisaged that allows continuous regeneration of the sacrificial agent.
Florida Solar Energy Center, a research institute of the University of the Central Florida, has proposed a class of efficient, solar driven thermochemical water splitting cycles, in which solar photonic energy is used for the production of hydrogen from an aqueous ammonium sulfite solution, while solar thermal energy is employed to produce oxygen. This unique approach of utilizing the total solar radiation spectrum may enhance the efficiency of solar to hydrogen energy conversion. Previously, a new class of solar hybrid thermo-photochemical water splitting cycles that utilize ammonium sulfite as a regenerable reagent for the production of hydrogen from water was described. One version of the cycle that incorporates a sub-cycle based on the transition metal oxides for the oxygen evolution is given below:(NH4)2SO3(aq)+H2O(l)→(NH4)2SO4(aq)+H2  (1)                (Photocatalytic, 25° C.)(NH4)2SO4(s)+MO(s)2NH3(g)+MSO4(s)+H2O(g)  (2)        (Thermolytic, 500° C.)MSO4(5)→SO2(g)+MO(s)+½O2  (3)        (Thermolytic, 900° C.)SO2(g)+2NH3(g)+H2O(l)→(NH4)2SO3(aq)  (4)        (Chemical absorption, 25° C.)where M=Zn, Fe, Mg, etc. (NH4)2SO3 is the electron donor reagent in the photocatalytic reaction (1). The ammonium sulfate, (NH4)2SO4, forms as a main product of the reaction (1) that is converted to SO2, ammonia and water via thermolytic reactions (2) and (3). Ammonium sulfite is regenerated by the absorptive reaction of SO2, NH3 and water, reaction step (4).        
In a closed water splitting cycle, the overall energy conversion efficiency depends on the efficiencies of both the H2 and O2 production steps. The thermolytic processes represented by reactions (3) and (4) are fairly efficient due to the high reaction temperatures involved. Therefore, the energy conversion efficiency of the photocatalytic H2 production step represented by the reaction (1) dictates the overall cycle efficiency for converting solar energy to that stored as chemical energy in hydrogen.
Although visible-light induced photocatalytic activity of CdS for H2 production from aqueous solutions of alkali metal sulfites and sulfides has been known since the early 1980s, in most cases, Pt (alone or in combination with other metals) was used as a cocatalyst with the aim of enhancing the H2 evolution rate. Recently, a number of composite cocatalysts with metal/oxide (core/shell) structure, such as Ni/NiO and Rh/Cr2O3, have been shown to impart higher catalytic activity than that of only metal or metal oxide cocatalysts.
Methods for the preparation of cocatalysts fall into two broad areas, namely: impregnation and photodeposition. In the former impregnation method, a high temperature activation process (300-500° C.) is often needed. The latter photodeposition technique has been primarily used for the deposition of noble metal particles. Neither of these two methods is particularly useful in preparing composite cocatalysts containing a metal oxide.
The method and system of the present invention provides a new Pd—Cr2O3 nanocomposite cocatalyst and a facile method for its preparation at room temperature. Unlike the impregnation and photodeposition methods, cocatalyst activation is not required Experimental results showed that the as-prepared cocatalyst shows high efficiency toward H2 evolution that is comparable to that obtained using much more expensive Pt based cocatalysts.